Gamma radiation-induced synthesis of novel PVA/Ag/CaTiO3 nanocomposite film for flexible optoelectronics

A flexible nanocomposite film based on polyvinyl alcohol (PVA), silver nanoparticles, and calcium titanate (CaTiO3) was synthesized using gamma radiation induced-reduction. Temperature-dependent structural, optical, DC electrical conductivity, electric modulus, and dielectric properties of PVA/Ag/CaTiO3 nanocomposite film were investigated. The XRD pattern proved the successful preparation of the nanocomposite film. Also, as the temperature increases, the average crystallite sizes of CaTiO3 and Ag nanoparticles decrease from 19.8 to 9.7 nm and 25 to 14.8 nm, respectively. Further, the optical band gap increased from 5.75 to 5.84 eV with increasing temperature. The thermal stability is improved, and the semiconductor behavior for PVA/Ag/CaTiO3 nanocomposite film is confirmed by thermal activation energy ΔE with values in the 0.11–0.8 eV range. Furthermore, the maximum barrier Wm value was found of 0.29 eV. PVA/Ag/CaTiO3 nanocomposite film exhibits a semicircular arc originating from the material’s grain boundary contributions for all temperatures. The optical, DC electrical conductivity, and dielectric properties of the PVA/Ag/CaTiO3 nanocomposite film can be suitable for flexible electronic devices such as electronic chips, optoelectronics, and energy storage applications.

Preparation of calcium titanate nanopowder. Calcium titanate powder was prepared from calcium carbonate (CaCO 3 ) and titanium dioxide (TiO 2 ) by solid-state reactions method. The raw materials were weighed by the stoichiometric 1:1 (Ca: Ti) molar ratio. Then, the powder was ground by pestle and mortar for 30 min and in a ball milling for 8 h. After the homogenization, mixtures were sintered in an air-resistive furnace at temperatures of 1000 °C for 2 h with a heating rate of 10 °C/h. Finally, the powder was manually crushed with a pestle and mortar to obtain a uniform CaTiO 3 powder.
Fabrication of PVA/Ag/calcium titanate nanocomposite. 0.5 g of AgNO 3 was added to 2 g of CaTiO 3 and 10 ml of isopropanol and stirred for 60 min using magnetic stirring. Then, the examined samples' solutions were irradiated with 50 kGy (dose rate of 0.8 kGy/h) at ambient conditions 28 . The radiation process was achieved using Co-60 gamma-cell sources [29][30][31] . As presented in previous work 26,32,33 , water radiolysis induced by gamma rays in an aqueous Ag + solution results in the generation of a significant number of highly effective reducing (hydrated electrons (e − aq ), hydrogen atoms (H⋅), and oxidizing radicals as HO⋅. Ag NPs are formed when silver ions are converted to silver atoms, which serve as individual nucleation centers, and subsequent coalescence forms Ag NPs. Isopropyl alcohol (isopropanol) may be used to scavenge the ⋅OH produced by radiation.
A solution of 6 wt% PVA was made by dissolving 6 g of PVA in 100 ml of deionized water at 70 °C. while stirring the mixture constantly until a homogenous solution had been achieved. After that, a specified concentration of Ag/CaTiO 3 was mixed with PVA solution for 2 h during stirring. The thick solution was cast onto a transparent glass plate using a film applicator and left to dry. After one week of drying in air at room temperature, the PVA/Ag/CaTiO 3 nanocomposite was scraped off of the glass plate, see Fig. 1. After that, the PVA/Ag/CaTiO 3 nanocomposite films were exposed to different temperatures (313, 323, 333, 343, 353, 363, and 373 K) for 30 min.
Characterization of PVA/Ag/CaTiO 3 nanocomposite. The crystal structure and phase analysis of pure CaTiO 3 , Ag/CaTiO 3 NPs, and PVA/Ag/CaTiO 3 nanocomposite were characterized via XRD Shimadzu 6000 with 40 kV and 30 mA and scanning rate of 8°/min as operating conditions. Fourier transforms infrared (FT-IR) spectroscopy has been employed to identify the functional groups in CaTiO 3 powder and PVA/Ag/ CaTiO 3 nanocomposite films (NICOLET iS10 model instrument). Moreover, high-resolution transmission electron microscopy (HR-TEM, (JEOL-JEM-100 CX)) was used to provide sufficient information on the particle size and the selected area electron diffraction (SAED) pattern of Ag/CaTiO 3 NPs. A scanning electron microscope (SEM, (JEOL JSM-5600 LV, Japan)), at variable vacuum without any coating at 12 kV accelerating voltage with a back-scatter detector, was also employed to develop surface images of PVA/Ag/CaTiO 3 nanocomposite to provide a clear insight into the morphology of the PVA/Ag/CaTiO 3 nanocomposite film surface. The energy-dispersive X-ray analysis spectra were also used to acquire the elemental composition and mapping pictures (EDX, JEOL JSM-5600 LV, Japan.). Also, TGA-50 Shimadzu, with a heating rate of 10°/min in an N 2 environment in the temperature range 293-873 K, was used to address the thermal stability of the PVA/Ag/CaTiO 3 nanocomposite. Using a UV-vis-NIR spectrophotometer (Jasco, V-570), we conducted the optical characteristics from

Results and discussion
Structural analyses.   www.nature.com/scientificreports/ indicating that the Ag NPs were successfully loaded on the surface of CaTiO 3 38 . The highly intense peaks of Ag NPs are due to the high amount of free silver in the composite 39 . A slight shift in the diffraction peaks of CaTiO 3 toward lower angles was observed; this could be attributed to the internal stress arising from the loading of Ag NPs to the lattice 40 . The broadening of the CaTiO 3 NPs peaks is decreased after Ag NPs loading, which increases the average size of CaTiO 3 NPs to 26.9 nm. The sharp diffraction peaks in both patterns of CaTiO 3 NPs and Ag/ CaTiO 3 NPs indicate the high crystalline nature of these samples 41 . Figure 3a shows a transmission electron microscope (TEM) image of Ag/CaTiO 3 NPs. This figure demonstrates that Ag/CaTiO 3 NPs are spherical NPs with a particle size between 11 and 30 nm. These results are consistent with the XRD calculations. The selected area electron diffraction (SAED) pattern of Ag/CaTiO 3 NPs, Fig. 3b, reveals bright spots at regular positions, which is evidence that the Ag/CaTiO 3 NPs have a crystalline structure. Figure 4 shows XRD diffraction patterns of PVA/Ag/CaTiO 3 nanocomposite at different temperatures (313, 323, 333, 343, 353, 363, and 373 K). The typical diffraction peak of PVA, which corresponds to the (101) plane, appeared along with the diffraction peaks of Ag/CaTiO 3 NPs. It is observed that the intensities of the diffraction peaks of Ag/CaTiO 3 NPs are severely decreased after the dispersion of the nanoparticles into the polymer matrix due to the higher content of PVA amorphous polymer, which reduces the crystallinity 42 . The average crystallite size for the most intense peak of CaTiO 3 NPs decreases from 19.8 nm at 313 K to 9.7 nm at 373 K. Furthermore, the diffraction peaks of Ag/CaTiO 3 NPs are shifted toward smaller angles after dispersing in the PVA matrix, indicating the intercalated structure owing to the formation of PVA/Ag/CaTiO 3 nanocomposite 43,44 . As the temperature increases from 313 to 373 K, the FWHM of the diffraction peaks increases, and then the average  www.nature.com/scientificreports/ crystallite size decreases from 25 to 14.8 nm for the intense peak of Ag NPs. However, at 353 K, the intensity of the diffraction peaks is increased, and the peaks become narrower; the FWHM decreased to 1.33, 0.36, and 0.31° for the most intense peaks of PVA, CaTiO 3 , and Ag, respectively, indicating improved crystallinity. This may be attributed to the glass transition temperature of PVA, at which the physical properties of a polymer nanocomposite change [45][46][47] . Figure 5 shows FTIR spectra of pure CaTiO 3 and Ag/CaTiO 3 NPs. The spectra show two strong bands at 3400 cm -1 and 3200 cm −1 that corresponded to vibration stretching in the O-H group that is suitable for the existence of hydroxyl group O-H, and the peak at 1721 cm −1 is attributed to the stretching vibrations of carbonyl (C=O) groups 48 . The bands at 1615 and 1375 cm −1 arise due to the stretching vibrations of C=C and bending vibrations of the carboxyl (C-OH) group. It's noticed that the two characteristic peaks at 534 and 434 cm −1 in the spectrum of CaTiO 3 NPs are related to the stretching and bending vibrations of Ti-O bonds 49 .
It can be observed that pure CaTiO 3 and Ag/CaTiO 3 NPs exhibited essential differences in the intensity of some peaks. Due to the effect of gamma irradiation, the intensities of peaks at 3440 and 1650 cm −1 of Ag/CaTiO 3 NPs significantly increased when compared to that of pure CaTiO 3 NPs, and there is a shift in some peaks to lower wavenumber in the spectrum of Ag/CaTiO 3 NPs which corresponding to bonding interaction of Ag and CaTiO 3 NPs which confirms the successful formation of Ag/CaTiO 3 NPs 50 . Figure 6 shows the FTIR spectra of PVA/Ag/CaTiO 3 nanocomposite at different temperatures (313, 323, 333, 343, 353, 363, and 373 K). The typical bands of PVA appeared along the spectra of Ag/CaTiO 3 . Where the broadening bands of PVA are due to O-H stretching vibration along the backbone of the polymer at 3278 cm −1 , the peak at 2922 cm −1 is due to the stretching vibration of the C-H alkyl group, and the peak at 1702 cm −1 corresponds to the stretching of the PVA acetate group's C=O bond. These bands decreased with increasing temperature and were related to the evaporation of water. The band at 1422 cm −1 is due to CH 2 symmetric bending; C-H wagging vibrations could describe the peak at 1308 cm −1 . The skeletal vibration of PVA corresponds to the peak at 820 cm −148 . It is noticed that the two characteristic peaks at 984 cm −1 and 1679 in the spectrum of PVA correspond to the CH 2 asymmetric stretching 51  This demonstrates that the nanoparticles interact with the PVA by the Van der Waal force, confirming the formation of PVA/Ag/CaTiO 3 nanocomposite 53 .
It is observed that with increasing the temperature, there is a noticeable change in the intensity of the bands at 3278 cm −1 and 2922 cm −1 for the sample at 353 K. This may be due to the glass transition temperature of pure PVA, which is approximately at 354.5 K 45 .
The morphology of the PVA/Ag/CaTiO 3 nanocomposite film was assessed using SEM images. The fabricated PVA/Ag/CaTiO 3 nanocomposite film consistently distributed Ag/CaTiO 3 NPs in the PVA polymer matrices, as seen in the SEM images (Fig. 7a,b). EDX spectra have been employed to validate the elemental composition of the PVA/Ag/CaTiO 3 nanocomposite film, as seen in Fig. 7c. The purity of the PVA/Ag/CaTiO 3 nanocomposite film is proven by Fig. 7c, which shows only the elemental peaks for C, Ti, Ag, Ca, and O and no other elements peaks. The mapping images (Fig. 8) showed that Ag/CaTiO 3 NPs were evenly distributed throughout the PVA matrix. www.nature.com/scientificreports/   Fig. 9a. The absorption band at 288 nm corresponds to the n-π* transition 54,55 , while the apparent absorption hump at 440 nm is attributable to the surface Plasmon resonance (SPR) of Ag NP, as has been reported in our earlier work 56 . Moreover, when the temperature rises, the absorption of the polymeric films has influenced significantly. As temperatures rise, a blue shift is seen in the position of the surface Plasmon peak. Hence, the optical bandgap of materials may be expected from absorption studies, which is crucial from the perspective of technological applications. As a result, since the optical characteristics of the PVA/Ag/CaTiO 3 nanocomposite may be directly associated with structural and electrical properties, they are critical for applications. The absorption coefficient α(λ) is related to the optical band gap E g via the following relation 57,58 : www.nature.com/scientificreports/ where B signifies band tailing, hν represents photon energy, and the power m specifies the transition. The energy gap of PVA/Ag/CaTiO 3 nanocomposite films in direct transition at various temperatures was evaluated by plotting (αhν) 2 against (hν), as can be seen in Fig. 9b. The direct band gap energy Eg of PVA/ Ag/CaTiO 3 nanocomposite at 313 K is found to be 5.75 eV. As the temperature increases, the direct band gap energy of PVA/Ag/CaTiO 3 nanocomposite films increases. By increasing the temperature, the optical band gap was increased to 5.84 eV at 373 K. This increase might be due to temperature changes influencing the electronic structure of the PVA chain. In other words, an increase in Eg values induces lattice defect and enhances the degree of electronic disorder in PVA/Ag/CaTiO 3 nanocomposite film, resulting in a loss in the crystallinity of Ag NPs. The exceptional decrease in Eg value at 323 K (5.72 eV) may be due to a change in crystallinity, as shown by XRD measurement 59 . Thermogravimetric analysis (TGA). Thermal stability is essential for using PVA/Ag/CaTiO 3 nanocomposite film in high temperatures in optoelectronic devices. The thermal stability of PVA/Ag/CaTiO 3 nanocomposite film was tested from 313 to 873 K at a constant rate of 10 K min −1 under a nitrogen atmosphere.
As seen in Fig. 10, the PVA/Ag/CaTiO 3 nanocomposite film's thermo-gravimetric analysis (TGA) curve exhibits three significant weight loss areas. The first area, which occurred at a temperature ranging from 343 to 451 K, has been associated with the evaporation of water that had been slightly adsorbed, and the weight loss of the film was around 4.409%. The second area occurs between 540 and 630 K and is caused by the decomposition of the PVA polymeric matrix. At this point, the film has lost around 49.39% of its weight. The reason for the weight loss that occurred during the third stage, which was correlated with 33.38% of the film at its highest temperature of 652-682 K, was that PVA chains split into several tiny fragments. At this point, the overall percentage of losing weight is around 99.1% 60 . The thermal stability of the PVA/Ag/CaTiO 3 nanocomposite is improved by mixing Ag/CaTiO 3 into the PVA matrix 51,61 .  www.nature.com/scientificreports/ Direct electrical conductivity. The direct electrical conductivity, denoted by the σ dc , is independent of the frequency and results from the free charges in the sample. The direct electrical conductivity σ dc can be expressed from the relation between the resistance R of the film and its length l and the area A, as follows 62 : The dc conductivity of PVA/Ag/CaTiO 3 nanocomposite film gradually increases with the increase in temperature, as shown in Fig. 11a. When the temperature rises, electrons from the valence band can jump to the conduction band, allowing free mobility between the two bands and improving the material's conductivity. The increased dc conductivity with the temperature reveals that the PVA/Ag/CaTiO 3 nanocomposite film exhibits semiconductor characteristics 63 .
This work describes the conduction mechanism of σ dc for PVA/Ag/CaTiO 3 nanocomposite film in terms of a variable range hopping mechanism (VRH). The following connections provide the foundation for this explanation 64 : where T 0 is the Mott temperature.
The VRH model's validity was tested for PVA/Ag/CaTiO 3 nanocomposite film by plotting ln (σ dc T 1/2 ) vs. T −1/4 as shown in Fig. 11b. The slope of the VRH curve was used in the calculation to get the value of T 0 . As a result of our findings, we have concluded that the transport model (VRH) may be the primary low-temperature transport mechanism. The obtained value of T 0 is 5.88 × 10 9 K and 6.87 × 10 9 K for PVA/Ag/CaTiO 3 nanocomposite film in the temperature regions R-I and R-II, respectively.
Dielectric measurements. One of the essential features of composite films is dielectric permittivity, which represents the material's propensity to polarize. It physically represents the more remarkable polarization generated in a material by an external field of specific strength. The complex permittivity describes and gives the dielectric characteristics 65 : where ε 0 , ε r , and ε i are the free space, real and imaginary parts of permittivity of complex dielectric constant.
It is generally known that ε r indicates the amount of electric energy stored in the material due to the applied alternating electric field. In addition, ε r depicts the strength of the dipole arrangement concerning the direction of the electric field 66 . Figure 12 depicts the dependency of the dielectric constant (ε r ) on the temperature for PVA/Ag/CaTiO 3 nanocomposite film at different frequencies (1.0 kHz to 1.0 MHz). This figure demonstrates that the ε r for PVA/Ag/CaTiO 3 nanocomposite film rises with temperature and is decreased with frequencies. We interpret this behavior in the following manner: as the temperature of PVA/Ag/CaTiO 3 nanocomposite film rises, the possibility of producing more charge carriers with high mobility (holes and ions) likewise rises 67 . Compared to the low temperature, the electric dipoles may efficiently align spontaneously. One possible explanation for the lower values of ε r seen at high frequencies is that the charges at the interfaces are unable to realign their direction in response to the intense alternating electric field. In addition, while performing at low frequencies, the interface charges have been given the necessary time to realign themselves and participate in the ε r 63 . Similarly, data on the dissipated energy in the PVA/Ag/CaTiO 3 nanocomposite film can be derived by graphing the imaginary component of the dielectric constant ε i against temperature at various frequencies (1 kHz to 1.0 MHz), as illustrated in Fig. 13. It demonstrates that ε i is independent of temperature but is inversely proportional to frequency, decreasing as the frequency increases. In summary, at these temperatures (313-373 K), the PVA/Ag/CaTiO 3 nanocomposite film interface charges have appropriate mobility and contribute significantly www.nature.com/scientificreports/ to the dissipated energy. Regarding frequency dependency, the interface charges may rearrange themselves at low frequencies but cannot at high frequencies 63,67 .
AC-conductivity and conduction mechanism. The AC conductivity σ ac provided through various mechanisms comprises inherent charge carriers, lattice energy, defects, and impurities. The AC conductivity σ ac measurements are performed to completely comprehend the conduction behavior of the parameters that may influence this mechanism of the samples. It is then considered to assess it for suitable applications [66][67][68] . Furthermore, if the σ ac of semiconductors refuses to obey the Arrhenius universal equation, it may be expressed as follows 69,70 : where E ac is the activation energy.
The relationship between ln σ ac and 1000/T for PVA/Ag/CaTiO 3 nanocomposite film is shown in Fig. 14a 67 . For a given temperature, the conductivity is constant and independent of frequency in the low-frequency zone. Furthermore, Ag/CaTiO 3 perovskite materials contain well-conductive grains surrounded by less conductive grain borders, and their activity is greater at lower frequencies. This effect leads to poor conductivity because of a modest electron jump in this location. When the frequency rises, the conductivity increases significantly. This is due to more charge carriers and more processes in the material for jumping charge carriers between consecutive sites. The phenomena then show dispersive behavior, yielding AC conduction conductivity σ ac 71,72 . The enhancement in conductivity with increasing temperature reveals that the conduction mechanism in the PVA/Ag/CaTiO 3 nanocomposite film has been thermally activated. For PVA/Ag/CaTiO 3 nanocomposite film,  www.nature.com/scientificreports/ the activation energy reduces as the temperature increases. The activation energy E ac of PVA/Ag/CaTiO 3 nanocomposite film has been computed from the slope of linear parts for specified frequencies and is observed to be in the range 0.11-0.8 eV (see Fig. 14b).
In addition, the value of the alternating current conductivity σ ac of the PVA/Ag/CaTiO 3 nanocomposite film could potentially be described as a function of the angular frequency ω 73,74 : where B would be a constant related to a given temperature and S is the frequency exponent that denotes the degree of interaction between mobile ions and lattices in the PVA/Ag/CaTiO 3 nanocomposite film 75 .
The relationship between log σ ac and log ω for PVA/Ag/CaTiO 3 nanocomposite film is seen in Fig. 15. The conductivity of PVA/Ag/CaTiO 3 nanocomposite film rises sharply with frequency. Furthermore, for most semiconductors, this change will occur at a certain frequency known as the hopping frequency, which improves with increasing temperature. Our PVA/Ag/CaTiO 3 nanocomposite film's high conductivity values suggest they could be utilized in electronic applications such as optoelectronics, electronic chips, and gas sensors.
The slopes of the straight lines in Fig. 16 at high frequencies are utilized to derive the exponent S. Figure 16 depicts the dependency of the frequency exponent, S, of PVA/Ag/CaTiO 3 nanocomposite film on temperature. The parameter S is essential in defining the conduction mechanism in PVA/Ag/CaTiO 3 nanocomposite film. According to Funke et al. 64 , if S is less than 1, the charge carriers suffer a transport displacement with a sudden hopping, and if S is more than 1, the species gets a located jump. In our study, PVA/Ag/CaTiO 3 nanocomposite film has S less than 1, resulting in a transport displacement with a sudden hopping for the charge carriers. As the temperature of a PVA/Ag/CaTiO 3 nanocomposite film rises, the exponent S decreases dramatically 67 . There is a suggestion that this behavior might be explained by the correlated barrier hopping (CBH) model. In the CBH σ ac = Bω s  www.nature.com/scientificreports/ model, the electrons react to the forces produced by an applied electrical field through jumping the potential barrier on their path from one hopping site to another. This takes place as a direct result of the electrons' potential to hop across the barrier 76 . According to the CBH model, the frequency exponent, S, can be expressed as 77 : where k B denotes Boltzmann's constant and W m is the height of the maximum barrier (the amount of energy required to get an electron out of its ground state towards its excited state). The height of the maximum barrier W m value was found of 0.29 eV.
The dielectric modulus. Studying electric modulus formalism is a practical approach to exploring the electrical transport mechanism and learning more about the relaxation process. The complex modulus for PVA/Ag/CaTiO 3 nanocomposite film can be represented using an equation 78 .
in which M r and M i denoted the complex modulus's real and imaginary components.  Figure 16. The dependency of frequency exponent, S, for PVA/Ag/CaTiO 3 nanocomposite film on temperature.  www.nature.com/scientificreports/ the material's grain boundary contributions, first on the low-frequency side and then on the higher-frequency side. Furthermore, each semicircular arc nearly overlaps the next with slight fluctuation for all temperatures, demonstrating an electrical relaxation development in the PVA/Ag/CaTiO 3 nanocomposite film 78 80 have reported that the crystallite size of ZnS increased from 4 to 10 nm as the annealing temperature rose from 300 to 500 °C. The extinction coefficient of PVA/ZnS nanocomposite improves by adding ZnS NPs at 300 °C and then reducing as the annealing temperature rises. The index of refraction increases for PVA/ZnS annealed at 300 °C and then declined as more annealed nano additives are added. Also, the direct energy gap values increase from 3.3 to 4.9 eV at 500 °C, and the indirect band gap increases from 2.3 to 4.7 eV for PVA/ZnS annealed at 500 °C.
Also, According to Sathish et al. 81 , the optical transmittance of PVA/Al 2 O 3 thin film was approximately 80% for the as-grown film, while it boosted with annealing temperature and the band gap energy (3.74-3.78 eV) reduced with annealing temperature. The dielectric constant values ranged between 8 and 16. The obtained values for the dielectric constant are greater than those of pure PVA. The values for dielectric loss have been determined to be between 0.1 and 0.6.

Conclusion
Herein, gamma radiation-induced synthesis of Ag/CaTiO 3 NPs and then dispersed in a PVA matrix. The temperature-dependent structural, optical, DC electrical conductivity, and dielectric characteristics of PVA/Ag/CaTiO 3 nanocomposite film were studied. As the temperature increased, the average crystallite sizes of CaTiO 3 and Ag NPs decreased from 19.8 to 9.7 nm and 25 nm to 14.8, respectively. The optical band gap increased from 5.75 to 5.84 eV at 373 K. Moreover, the increase of the dc conductivity with the temperature shows that the PVA/Ag/ CaTiO 3 nanocomposite film exhibits a semiconductor behavior. The frequency exponent, S, of PVA/Ag/CaTiO 3 nanocomposite film, gradually decreases as the temperature increases and is less than 1. Further, the maximum barrier W m value is around 0.29 eV. This unique optical, DC electrical conductivity and dielectric properties of PVA/Ag/CaTiO 3 nanocomposite film reveal that it can be used for flexible electronic devices.

Data availability
All data generated or analysed during this study are included in this published article. www.nature.com/scientificreports/